Electrolytic sealing of anodized aluminum surfaces



Patented May 3, 1949 ELECTROLYTIC SEALING OF ANODIZED ALUMINULI SURFACES James F. Mason, Glen Cove, N. Y., and Austin H. Beebe, Jr., Ann Arbor, Mich., assignors to Reynolds Metals Company, Inc.,

Richmond,

Va., a corporation of Delaware No Drawing. Application October 25, 1945, Serial No. 624,608

2 Claims.

Our invention relates to improvements in the sealing of oxide films produced on aluminum surfaces by anodic exposure in appropriate electrolytes.

For many purposes aluminum surfaces are protected by an oxide film produced on the surface by exposure, as an anode, in an acid electrolyte, an aqueous solution of sulfuric acid, oxalic acid, chromic acid or sulfamic acid for example. Hard and adherent films can thus be produced, but the films thus produced are not impervious and for this reason are not satisfactory for many purposes. The operation of forming such films is commonly termed anodizing and the aluminum surface thus protected is commonly termed anodized.

Hitherto, the sealing of such films, that is the treatment of such films to render them impervious, has been effected by immersing the anodized aluminum surface in water at approximately boiling point in open tanks. The usual explanation of the conversion of the pervious oxide film to a substantially impervious film thus affected is hydration and consequent swellin of the particles of oxide constituting the original coating.

We have now found that effective sealing can be accomplished at temperatures lower than those hitherto believed to be necessary. The advantages of lower temperature processing, particularly from the standpoint of the operator, will be obvious. The quality of the final film produced by our new process is at least as good as that of films produced by older processes although we have some reason to believe that the film produced by our process may differ in constitution from the films produced by the prior practices just described.

In carrying out our invention, we seal the anodized aluminum surface to be processed by anodically exposing it in an aqueous solution of sodium phosphate having a pH of about 4.1-5.0 at a temperature of 65-85 C. with a current density of about 1-2 amperes per square foot of anodized surface until scaling is complete. An exposure time of about 15-30 minutes is usually advantageous. While the voltage requisite to maintain the appropriate current density will vary with different equipment, we have found a voltage of 105-120 to be satisfactory. The pH of the treating solution is easily adjusted by additions of either phosphoric acid or sodium hydroxide as may be required. This sealing treatment is applicable to the oxide films produced by any of the anodizing operations above listed.

The pH range and the concentration range just given as approximations are in fact optimum ranges in our experience. If the pH and concentration of the treating solution become lower or higher than the indicated range, the sealing becomes progressively less effective. The temperatures above about 65 C. are generally useful in carrying out our process. Also, exposure time upwards of about 15 minutes is generally useful in carrying out our process although economy dictates that the exposure time will appropriately be limited to that necessary to affect the sealing.

So far, the results we have attained seem to be peculiar to phosphates. We have tried a variety of other salts. With the exception of phosphates, the salts we have tried generally have been either ineffective to produce sealing or they have produced pitting although some salts have appeared to produce sealing without pitting within very narrow ranges of current density and temperature.

While we do not predicate our invention upon any particular hypothesis, we do wish to make two notes. The effect appears to involve electrolysis in the sense that the sealing requires that current pass through the oxide film on the anodized surface. Immersion of the anodized surface in the phosphate solution at the same temperature without electrolysis appears to be ineffective. Also, we have found that the electrolytically sealed film contains phosphate using the molybdate method of analysis. We suspect that the phosphate present is aluminum phosphate formed at the aluminum surface, since the current must pass through the film to make the sealing effective, and then forced into the pores in the oxide film.

The sealed film produced by our process is tough, hard, adherent and impervious. It may be colored by use of any of the conventional dyeing techniques applied to the anodized surface prior to application of the sealing treatment.

We claim:

1. In sealing anodized aluminum surfaces, the improvement which comprises anodically treating the anodized aluminum surface in an aqueous solution of sodium phosphate having a pH of about 4.15.0 at a temperature of about 65-85 C. with a current density of about 1-2 amperes per square foot of anodized surface until sealed.

2. In sealing anodized aluminum surfaces, the improvement which comprises anodically treating the anodized aluminum surface in an aqueous solution of sodium phosphate having a pH of about 4.1-5.0 at a temperature of about 65-85 C.

with a. current density of about 1-2 amperes per square foot of anodized surface for about 15-30 minutes. Number JAMES F. MASON. 2,079,516 AUSTIN H. BEEBE, JR. 5 2, ,954"

2,313,755 REFERENCES CITED 2,322,205

The following references are of record in the' file of this patent:

UNITED STATES PATENTS Name Date Lilienfeld Mar. 4, 1937 Edwards Aug. 16, 1943 Loose Mar. 16, 1943 DeLong June 22, 1943 

